Benzo[c]thiophene-C60 diadduct: An electron acceptor for p-n junction organic solar cells harvesting visible to near-IR light

Article abstract of DOI:10.1002/asia.201200698

We synthesized a new 56-π-electron fullerene derivative through a Diels-Alder cycloaddition of benzo[c]thiophene that featured a relatively low temperature, closer to stoichiometric use of the diene, and easy product purification. The 56-π-electron benzo[c]thiophene diadduct (BTCDA) has a LUMO energy level of 0.09 to 0.18 eV higher than that of 58-π-electron fullerenes, and therefore, the BTCDA-based organic photovoltaic device exhibited a higher open-circuit voltage and power-conversion efficiency (PCE). When used with a binary-donor system, including visible-light-harvesting tetrabenzoporphyrin (BP) and near-IR-harvesting titanyl phthalocyanine (TiOPc), the device had a PCE that was 1.5-3 times higher (2.8 %) than that for devices with BP or TiOPc alone because the binary-donor device can utilize light between λ=350 and 950 nm. Copyright

Full text of DOI:10.1002/asia.201200698

DOI: 10.1002/asia.201200698
Benzo[c]thiophene–C60 Diadduct: An Electron Acceptor for p–n Junction
Organic Solar Cells Harvesting Visible to Near-IR Light
Yonggang Zhen,[a] Naoki Obata,[a, b] Yutaka Matsuo,*[a] and Eiichi Nakamura*[a]
Abstract: We synthesized a new 56-p-
electron fullerene derivative through
a Diels–Alder cycloaddition of ben-
zo[c]thiophene that featured a relatively
low temperature, closer to stoichiomet-
ric use of the diene, and easy product
purification. The 56-p-electron ben-
zo[c]thiophene diadduct (BTCDA) has
tron fullerenes, and therefore, the
BTCDA-based organic photovoltaic
device exhibited a higher open-circuit
voltage and power-conversion efficien-
cy (PCE). When used with a binary-
donor system, including visible-light-
harvesting tetrabenzoporphyrin (BP)
and near-IR-harvesting titanyl phthalo-
cyanine (TiOPc), the device had a PCE
that was 1.5–3 times higher (2.8%)
than that for devices with BP or TiOPc
alone because the binary-donor device
can utilize light between l=350 and
950 nm.
Keywords: cycloaddition · electro-
chemistry
· fullerenes · organic
a
LUMO energy level of 0.09 to
photovoltaics · phthalocyanines
0.18 eV higher than that of 58-p-elec-
Introduction
We report herein the synthesis and device performance of
a benzo[c]thiophene–C60 diadduct (BTCDA) synthesized in
54% yield and purified easily by column chromatography.
The introduction of a sulfur atom into the BTCDA molecule
Fullerene derivatives have found widespread application in
organic solar cells as excellent electron acceptors owing to
their spherical shapes for three-dimensional charge trans-
port and satisfactory phase separation.[1] Two recent topics
in the area are the harvesting of near-IR (NIR) light by en-
hancing the light absorbance of electron donors[2,3] and the
increase of the open-circuit voltage (VOC) of the organic
photovoltaic (OPV) devices through control of the energy
level of the electron donors and acceptors.[4,5] In this con-
text, 56-p-electron fullerene derivatives have attracted con-
siderable interest.[6,7] A Diels–Alder diadduct of fullerene,
an indene–C60 diadduct, is one such 56-p-electron compound
that has been widely examined for OPV devices.[7a,b] Such
devices tend to show higher VOC and power conversion effi-
ciency (PCE) than those using the standard material, [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM).[1a,8] Despite
its popularity, the indene diadduct suffers from some syn-
thetic problems: the use of a large excess of a diene reagent,
high reaction temperature, and difficulty of purification be-
cause of the similar physical properties of the multiadducts
and regio- and stereoisomers that concomitantly form.
À
was expected to afford S S noncovalent intermolecular
bonding interactions, which can improve the charge-trans-
porting properties of the electronic devices. A p–n junction
OPV device using a binary-donor layer (tetrabenzoporphyr-
in (BP; absorbing up to 700 nm) and titanyl phthalocyanine
(TiOPc; up to 950 nm)) had a PCE of 2.8%, which was
about 1.5–3 times larger than that obtained by the use of BP
or TiOPc alone, and 20% higher than the performance
(2.4%) of the similar binary-donor device using 1,4-bis(di-
methylphenylsilylmethyl)[60]fullerene (SIMEF) as an ac-
ceptor.[3b] The VOC value increased by 0.09 V, which corre-
sponded very well to the higher LUMO value for BTCDA
than that for SIMEF. The short-circuit current density (JSC)
and fill factor (FF) also contributed to the increase in PCE.
A comparison of device performances when using benzo[c]-
thiophene–C60 monoadduct BTCMA, diadduct BTCDA,
and triadduct BTCTA as acceptors for devices with BP indi-
cated that VOC increased predictably from 0.46 to 0.66 to
0.79 V in accordance with the increase in the LUMO level
from À3.79 to À3.65 to À3.52 eV, respectively.
[a] Dr. Y. Zhen, Dr. N. Obata, Prof. Y. Matsuo, Prof. E. Nakamura
Department of Chemistry, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)-3-5800-6889
Results and Discussion
Synthesis and Structural Characterization
E-mail: matsuo@chem.s.u-tokyo.ac.jp
nakamura@chem.s.u-tokyo.ac.jp
Benzo[c]thiophene 4 was prepared from a,a-dibromo-o-
xylene (1) in three steps.[9] The Diels–Alder reaction be-
tween benzo[c]thiophene (4 equiv) and C60 (1 equiv) in o-di-
chlorobenzene (ODCB) proceeded smoothly at 608C to
afford a mixture of BTCMA, BTCDA, and BTCTA, as
shown in Scheme 1. After silica-gel column chromatography
[b] Dr. N. Obata
Mitsubishi Chemical Group Science and
Technology Research Center, Inc.
1000 Kamoshida-cho, Yokohama 227-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200698.
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FULL PAPER
Optical and Electrochemical
Properties
The UV/Vis absorption spectra
of benzo[c]thiophene–C60 ad-
ducts are shown in Figure 2.
BTCMA exhibited a character-
istic sharp absorption peak at
431 nm,[10] and both BTCDA
and BTCTA showed higher ab-
sorbance in the region of 400–
500 nm
than
BTCMA.
BTCMA and BTCDA absorb
much stronger at about 700 nm
than BTCTA.
The cyclic voltammogram
for BTCMA showed three re-
Scheme 1. Synthesis of benzo[c]thiophene–C60 monoadduct BTCMA, diadduct BTCDA, and triadduct
BTCTA.
versible
reduction
waves,
(eluent: CS2/n-hexane=1:1), BTCMA, BTCDA, and
BTCTA were separated in 3.5, 54, and 27% yields, respec-
tively, without the use of preparative HPLC or repeated
silica-gel column chromatography, which is necessary for the
synthesis of the indene C60 adducts. The reaction took place
at a lower reaction temperature and with reduced amounts
of benzo[c]thiophene at the expense of a longer reaction
time.
As expected, the reaction produced several isomers for
BTCDA and BTCTA, as confirmed by 1H NMR spectra
(Figure 1). BTCDA and BTCTA were more soluble in tolu-
ene, chloroform, chlorobenzene, and THF than BTCMA,
which is insoluble in common organic solvents except for
ODCB and CS2 and hence can be readily removed from the
product mixture. The solubility of BTCDA was more than
1 wt% in toluene and chlorobenzene.
Figure 1. 1H NMR spectra of benzo[c]thiophene–C60 adduct BTCMA in
CDCl3/CS2 (1:1) (bottom), BTCDA (middle), and BTCTA (top) in
CDCl3.
Abstract in Japanese:
Figure 2. UV/Vis absorption spectra of benzo[c]thiophene–C60 adducts in
dichloromethane at room temperature. Inset shows the absorption spec-
tra in the region of 500–800 nm.
whereas those for BTCDA and BTCTA showed two reversi-
ble reduction waves in a negative potential range from À0.5
to À3.0 eV versus ferrocene (Fc/Fc+; Figure 3). Notably, the
first half-wave reduction potential was much more negative
from BTCMA to BTCDA to BTCTA because of the re-
duced p-conjugation length on the fullerene unit. From the
first half-wave reduction potentials, we estimated LUMO
levels of À3.79 eV for BTCMA, À3.65 eV for BTCDA, and
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Benzo[c]thiophene–C60 Diadduct
Table 2. Photovoltaic properties of BP-based p–n junction photovoltaic
devices with different acceptors under AM 1.5 illumination at
100 mWcmÀ2
.
Active layer
VOC [V]
JSC [mAcmÀ2
]
FF
PCE [%]
BP/BTCMA
BP/BTCDA
BP/BTCTA
0.46
0.66
0.79
0.76
0.70
0.61
0.54
3.2
3.3
2.8
5.6
8.2
7.7
9.4
0.39
0.47
0.30
0.42
0.48
0.50
0.45
0.6
1.0
0.7
1.8
2.8
2.4
2.3
TiOPc/BTCDA
BP/TiOPc/BTCDA
BP/TiOPc/SIMEF[a]
BP/TiOPc/PCBM[a]
[a] OPV devices with PCBM or SIMEF as the acceptor were character-
ized in our previous work.[3b]
Figure 3. Reductive cyclic voltammograms of benzo[c]thiophene–C60 ad-
ducts compared with those for PCBM and SIMEF. Measurements were
performed on a solution in THF containing [Bu4N]
ACHTUNGRTEN[NNUG ClO4] (0.1m) as a sup-
porting electrolyte at 258C with a scan rate of 0.1 VsÀ1
.
À3.52 eV for BTCTA. In contrast to PCBM and SIMEF, the
LUMO level increased by 0.18 and 0.09 eV, respectively, for
BTCDA, as shown in Table 1.
Table 1. Electrochemical properties of adducts relative to PCBM and
SIMEF.
LUMO[a] [eV]
red1
1=2
red2
1=2
Compound
E
[V]
E
[V]
BTCMA
BTCDA
BTCTA
SIMEF
PCBM
À1.01
À1.15
À1.28
À1.06
À0.97
À1.59
À1.72
À1.90
À1.63
À1.54
À3.79
À3.65
À3.52
À3.74
À3.83
Figure 4. Current density–voltage (J–V) curves of OPV devices with BP/
BTCDA, TiOPc/BTCDA, and BP/TiOPc/BTCDA as the active layers
under AM 1.5 illumination at 100 mWcmÀ2
.
[a] Values from the vacuum level were estimated by using the following
equation: LUMO level=À(Er1e=d21+4.80).[5a, 11]
dye with crystal-polymorph-dependent optoelectronic prop-
erties. The phase II form of TiOPc shows a particularly ex-
tensive and broad visible-to-NIR absorption and good p-
type semiconductivity. There are several methods to induce
the phase transition from I to II, including solvent vapor ex-
posure,[15] thermal annealing,[16] organic molecular beam
deposition,[17] supersonic molecular beam epitaxy,[18] and
substrate induction.[19] We recently found that treatment of
the TiOPc thin film of the device with toluene and chloro-
benzene was a very efficient way to transform phase I into
phase II in thin films.[3b] The HOMO of BTCDA in its film
state was estimated to be À5.8 eV by using a photoemission
yield spectroscopy (PYS)[20,21] measurement. Together with
the reported HOMO and LUMO levels of BP and TiOPc,[3-
Photovoltaic Device Performance
First, to evaluate the fundamental OPV performance of
BTCDA and congeners, we fabricated p–n OPV devices
with BP[5c,12] as an electron donor and the new fullerene de-
rivative as an electron acceptor (device configuration:
indium tin oxide (ITO)/poly(3,4-ethylenedioxylenethio-
phene)–polystyrene sulfonic acid (PEDOT:PSS)/BP/fuller-
ene/2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthro-
line (NBphen; 7 nm)/Al; see Figure 5b below); the results
are summarized in the top three rows of Table 2. The device
with BTCDA as the acceptor exhibited a PCE of 1.0%
(VOC =0.66 V; JSC =3.3 mAcmÀ2; FF=0.47); this was much
higher than that for the devices with BTCMA and BTCTA
as the acceptor materials (Figure 4 and Table 2). Having
a LUMO level of 0.14 eV lower than that of BTCDA, and
b,5a]
the energy levels of the active layers when using BP/
TiOPc/BTCDA can be drawn as shown in Figure 5a.
In an attempt to harvest both visible and NIR light, p–n
heterojunction OPV devices with TiOPc/BTCDA as the
active layer were fabricated and compared with the visible-
light-absorbing BP/BTCDA device and the NIR-absorbing
TiOPc/BTCDA device. As shown by the external quantum
efficiency (EQE) curve in Figure 6, the TiOPc/BTCDA
device (squares) uses short- and long-wavelength light ex-
tending to 900 nm, which is complementary to the BP/
BTCDA device (circles), which uses light up to 700 nm. The
overall efficiency of the TiOPc/BTCDA device therefore
hence,
a lower offset of frontier molecular orbitals
(EDHOMOÀEALUMO), BTCMA had a VOC value as low as 0.46 V.
BTCTA has a higher VOC because of its higher LUMO
level; it has lower JSC and FF performance, probably be-
cause of the disordered molecular alignment in the active
layer with the three randomly oriented organic addends.[7e,13]
To utilize NIR light, which accounts for over 40% of total
solar energy, we chose TiOPc as a p-type layer for device
structure optimization.[3,14] TiOPc is an interesting nonplanar
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Y. Matsuo, E. Nakamura et al.
FULL PAPER
Experimental Section
General
All reactions with air- or moisture-sensitive compounds were carried out
by using standard Schlenk techniques or a glovebox under argon or nitro-
gen atmospheres. Flash silica-gel column chromatography was performed
on silica gel 60N (Kanto Chemical, spherical and neutral, 140–325 mesh).
NMR spectra were measured with a JEOL ECA-500 (500 MHz) spec-
trometer. Spectra are reported in parts per million from internal tetrame-
1
thylsilane (d=0.00 ppm) for H NMR spectroscopy, from residual solvent
(e.g., d=77.00 ppm for chloroform) for 13C NMR spectroscopy. Cyclic
voltammetry (CV) was performed by using a HOKUTO DENKO HZ-
5000 voltammetric analyzer. All measurements were carried out in a one-
compartment cell under argon, equipped with a glassy-carbon working
electrode, a platinum wire counter electrode, and an Ag/Ag+ reference
electrode. Measurements were performed in a solution of THF contain-
ing tetrabutylammonium perchlorate (0.1m) as a supporting electrolyte
at 258C with a scan rate of 0.1 VsÀ1. All potentials were corrected against
Fc/Fc+. UV/Vis absorption was measured on a Jasco V-570 spectrometer.
PYS was measured by using a PCR-101 instrument (Sumitomo Heavy In-
dustries Advanced Machinery).
Figure 5. OPV device configurations. a) Energy diagram of BP, TiOPc,
and BTCDA. b) The p–n heterojunction OPV device structure with BP
or TiOPc as the single p-type layer. c) OPV device structure with
a TiOPc interlayer between the BP and BTCDA layers.
Materials
Unless otherwise noted, all materials were purchased from Tokyo Kasei,
Aldrich, and other commercial suppliers, and used after appropriate pu-
rification before use. Anhydrous dichlorobenzene was purchased from
WAKO Pure Chemical and purified by a solvent purification system
(GlassContour)[22] equipped with columns of activated alumina and sup-
ported copper catalyst (Q-5) prior to use.
Figure 6. EQE curves of OPV devices with BP/BTCDA, TiOPc/BTCDA,
and BP/TiOPc/BTCDA as the active layers under AM 1.5 illumination at
Synthesis of Benzo[c]thiophene 4
100 mWcmÀ2
.
1,3-Dihydrobenzothiophene (2) was prepared according to a literature
method:[9c,d] A mixture of a,a’-dibromo-o-xylene (1; 3.0 g, 11.4 mmol),
sodium sulfide (7.5 g, 31.2 mmol), and benzyltriethylammonium chloride
(36 mg, 0.158 mmol) in CH2Cl2/H2O (1:1, 10 mL) was stirred in the dark
at room temperature for 15 h. The organic layer was separated, washed
several times with water, dried with sodium sulfate, concentrated, and
distilled at 808C under 0.8 torr pressure to give 2 as a colorless oil
(960 mg, 62%). 1H NMR (500 MHz, CDCl3): d=4.27 (s, 4H; CH2S),
has an increased PCE of 1.8% (VOC =0.76 V, JSC
5.6 mAcmÀ , FF=0.42; see Figure 4 and Table 2).
=
2
To our great satisfaction, the performance of the binary-
donor device BP/TiOPc/BTCDA was the sum of the BP-
and TiOPc-based devices, utilizing the whole range of light
between 350 and 950 nm, as shown by the EQE curve in
Figure 6. As a consequence, this device showed a PCE value
of 2.8% (VOC =0.70 V; JSC =8.2 mAcmÀ2; FF=0.48; see
Figure 4 and Table 2), which was much higher than that of
devices with a single-donor layer (shown in the top four
rows of Table 2). The BP/TiOPc/BTCDA device had
a higher PCE than the same binary-donor device with
SIMEF or PCBM mainly because of the higher VOC value,
which in turn originated from the higher LUMO level of
BTCDA than those of the SIMEF and PCBM acceptors.
7.20–7.24 ppm (m, 4H; Ar H). 1H NMR data corresponded well with
À
that reported in the literature.[9b,c]
1,3-Dihydrobenzo[c]thiophene-2-oxide (3) was prepared according to
a literature method:[9a,b] Compound 2 (817 mg, 6.00 mmol) was added
dropwise to a 50% aqueous stirred solution (30 mL) of sodium periodate
(1.28 g, 6.00 mmol) in methanol. After being stirred for 12 h at room tem-
perature, the reaction mixture was filtered to remove inorganic salts, con-
centrated, and separated by silica-gel column chromatography with ethyl
acetate to afford 3 as a white solid (890 mg, 97%). 1H NMR (500 MHz,
CDCl3): d=4.15 (d, J=16 Hz, 2H; -CHSO), 4.30 (d, J=16.1 Hz, 2H;
-CHSO), 7.32–7.37 ppm (m, 4H; Ar H). 1H NMR data corresponded
À
well with that reported in the literature.[9b]
Benzo[c]thiophene 4 was prepared according to a literature method:[9a,b]
A mixture of 3 (800 mg, 5.26 mmol) and grade I neutral alumina (2.0 g)
was heated for 2 h under 25 torr pressure at 1308C in a sublimer to give
4 as a white solid (607 mg, 86%). 1H NMR (500 MHz, CDCl3): d=7.04
(dd, J=2.9, 6.9 Hz, 2H; 5-,6-H), 7.60 (dd, J=2.9, 6.9 Hz, 2H; 4-,7-H),
7.65 ppm (s, 2H; 1-,3-H). 1H NMR data corresponded well with that re-
ported in the literature.9b
Conclusion
New benzo[c]thiophene–C60 adducts were synthesized
through [4+2] Diels–Alder cycloaddition reactions. By using
a combination of BP and TiOPc as p-type layers and
BTCDA as an n-type layer, the p–n junction OPV per-
formed much better (PCE=2.8%) than devices with
a donor alone (PCE=1.0 to 1.8%). A large VOC value and
broad absorption covering near-UV to NIR light contribut-
ed to the improved efficiency.
Synthesis of Benzo[c]thiophene–C60 Adducts
The synthetic route is shown in Scheme 1. Benzo[c]thiophene (537 mg,
4.00 mmol) and C60 (721 mg, 1.00 mmol) were dissolved in ODCB
(50 mL) and heated to 608C for 17 h. The reaction solution was diluted
with methanol (300 mL) to give a brown precipitate. After filtration, the
solid was purified by silica-gel column chromatography (eluent: CS2/
hexane=1:1) to afford BTCMA (30 mg, 3.5%), BTCDA (530 mg, 54%),
and BTCTA (300 mg, 27%).
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Benzo[c]thiophene–C60 Diadduct
BTCMA: 1H NMR (500 MHz, CDCl3/CS2 =1:1): d=5.94 (s, 2H), 7.35–
7.37 (m, 2H), 7.55–7.57 ppm (m, 2H); 13C NMR (125 MHz, CS2): d=
66.7, 77.7, 122.1, 127.0, 137.5, 138.8, 139.2, 139.7, 141.2, 141.7, 141.9,
142.3, 142.3, 144.0, 144.3, 144.7, 145.0, 145.1, 145.3, 145.7, 145.8, 146.0,
146.1, 146.9, 153.3, 155.0; elemental analysis calcd (%) for C68H6S: C
95.54, H 0.71, S 3.75; found: C 95.62, H 0.89.
BTCDA: 1H NMR (500 MHz, CDCl3): d=4.95–6.25 (m, 4H), 7.12–
7.79 ppm (m, 8H); 13C NMR (125 MHz, CDCl3/CS2 =1:4): d=66.7, 66.8,
67.0, 67.3, 67.4, 67.5, 67.6, 67.9, 68.0, 68.5, 75.9, 76.4, 76.5, 76.6, 76.7, 76.8,
77.0, 121.7, 121.8, 121.9, 122.0, 122.1, 122.2, 122.3, 126.5, 126.6, 126.7,
126.8, 126.9, 127.0, 127.1, 132.4, 133.5, 133.8, 135.3, 135.8, 135.9, 136.2,
136.5, 136.8, 136.9, 137.6, 137.8, 138.1, 138.2, 138.8, 139.1, 139.3, 139.6,
140.0, 140.5, 140.7, 141.2, 142.0, 142.5, 143.2, 143.6, 143.9, 144.2, 144.3,
144.4, 144.7, 145.0, 145.3, 145.4, 145.5, 145.7, 145.8, 145.9, 146.0, 146.1,
146.5, 147.1, 147.5, 147.6, 147.8, 148.0, 148.1, 148.8, 149.9, 150.3, 150.6,
150.7, 151.6, 151.8, 151.9, 152.1, 152.3, 152.4, 152.6, 152.9, 153.1, 153.3,
153.5, 153.6, 153.7, 153.8, 153.9, 154.3, 154.4, 154.5, 154.6, 155.3, 155.7,
155.8, 155.9, 156.1, 156.4, 157.2, 157.8, 157.9, 158.1, 158.2, 159.0, 159.1,
159.2, 159.5 ppm; elemental analysis calcd (%) for C76H12S2: C 92.29, H
1.22; found: C 92.52, H 1.48.
The two-donor (BP/TiOPc)-based heterojunction OPV devices were fab-
ricated by use of solvent-processed BP and vacuum-deposed TiOPc. The
ITO, PEDOT:PSS, BP, and TiOPc layers were fabricated as described
above. We fabricated the device with diadduct BTCDA (0.5 wt% in
chlorobenzene) by spin-coating to give a thickness of about 25 nm.
NBphen (7 nm) was deposited in vacuum (3ꢁ10À4 Pa) on top of active
layers as an exciton blocking layer, followed by the deposition of an alu-
minum electrode (Al, 80 nm) in vacuum, and thermal annealing at 808C.
The thickness of spin-coated layers was determined by using a Dektak
6M stylus profiler. All devices were encapsulated in a glove box under
a nitrogen atmosphere. The photocurrent of the fabricated OPV devices
was investigated with a sweeping voltage by using a Keithley 2400 source
measurement unit controlled by a computer under simulated solar light
using an AM1.5G light source with 100 mWcmÀ2 intensity. The incident
light intensity was calibrated to one sun (100 mWcmÀ2) with a standard
Si photodiode (BS-520, Bunkoukeiki, Japan). The current density versus
voltage (J–V) characteristics were measured for an area of 0.04 cm2.
EQE was measured under constant power generated by monochromat-
ized photons using a xenon or halogen lamp.
BTCTA: 1H NMR (500 MHz, CDCl3): d=4.95–6.01 (m, 6H), 6.98–
7.67 ppm (m, 12H); 13C NMR (125 MHz, CDCl3/CS2 =1:4): d=65.8, 65.9,
66.3, 66.5, 66.7, 66.8, 66.9, 67.0, 67.1, 67.5, 67.7, 67.9, 68.1, 68.3, 75.2, 75.5,
75.6, 75.8, 76.0, 76.1, 121.6, 121.7, 121.8, 121.9, 122.0, 122.1, 122.2, 122.3,
126.4, 126.5, 126.6, 126.7, 126.8, 126.9, 135.0, 136.3, 137.4, 137.5, 137.7,
138.8, 139.0, 140.0, 140.4, 140.5, 140.6, 140.7, 140.8, 140.9, 141.0, 141.1,
141.2, 141.3, 141.9, 142.0, 142.1, 142.2, 142.5, 143.5, 143.9, 144.3, 144.4,
145.1, 145.2, 145.3, 145.4, 145.5, 145.6, 145.7, 145.8, 145.9, 146.0, 146.1,
146.2, 146.3, 146.5, 148.1, 148.3, 148.6, 148.9, 151.0, 151.3, 151.9, 153.0,
154.1, 154.2, 154.4, 154.5, 154.7, 154.8, 154.9, 155.1, 155.2, 155.3, 155.5,
155.7, 156.1, 156.2, 156.3, 156.4, 156.5, 156.6, 156.8, 157.0, 157.8, 157.9,
158.1, 158.2, 158.4, 158.5, 159.8, 159.9, 160.0, 160.1, 160.9, 161.1, 161.3,
161.7, 161.9, 162.0, 162.2 ppm; elemental analysis calcd (%) for C84H18S3:
C 89.82, H 1.62; found: C 90.05, H 1.89.
Acknowledgements
We thank Dr. Hideyuki Tanaka and Dr. Lars Mattias Andersson (The
University of Tokyo) for helpful discussions on the data analysis. This
study was supported by the Funding Program for Next-Generation
World-Leading Researchers (Y.M.) and the Strategic Promotion of Inno-
vative Research and Development from the Japan Science and Technolo-
gy Agency (E.N.).
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Device Fabrication and Characterization
The p–n heterojunction OPV devices with BP as the p layer and ben-
zo[c]thiophene–C60 adducts as the n layer were fabricated by means of
the following procedure. A 145 nm thick, patterned indium tin oxide
(ITO) glass with a sheet resistance of 8 W/square was used as a substrate.
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fabricated the device with BTCDA (0.5 wt% in chlorobenzene) by spin-
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aluminum electrode were deposited as described for the previous proce-
dure.
Chem. Asian J. 2012, 00, 0 – 0
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Received: July 31, 2012
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ÝÝ These are not the final page numbers!

FULL PAPER
Organic Photovoltaics
Yonggang Zhen, Naoki Obata,
Yutaka Matsuo,*
Eiichi Nakamura*
&&&& — &&&&
Benzo[c]thiophene–C60 Diadduct: An
Electron Acceptor for p–n Junction
Organic Solar Cells Harvesting Visible
to Near-IR Light
Two donors are better than one: A p–
n junction photovoltaic device with
benzo[c]thiophene–C60 diadduct
(BTCDA) exhibited an improved
power conversion efficiency of 2.8%
and was capable of harvesting visible
to near-IR light through the insertion
of titanyl phthalocyanine (TiOPc)
between the tetrabenzoporphyrin (BP)
and BTCDA layers (see scheme;
ODCB=o-dichlorobenzene).
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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These are not the final page numbers! ÞÞ